Improvement of yields of common beans (Phaseolus vulgaris L.) by increased nitrogen fixation in soils with low phosphorus levels

Farmers in parts of Latin America and Africa could increase bean production on poor soils if bean cultivars capable of maintaining a sufficient level of N₂ fixation at low P were identified and disseminated. In the present study 51 Phaseolus vulgaris L. genotypes each inoculated with the Rhizobium tropici strain UMR1899 were screened for growth and N₂ fixation at 5 and 140 μM phosphorus (P) supplied as rock phosphate to a peat moss growth medium. Two genotypes (ANT22 and E295) with good growth and N₂ fixation and one (G19833) with poor growth at low soil P were identified. ANT22 and E295 had greater nodule mass, higher nitrogenase activity and accumulated more plant P and N than G19833, and maintained a higher nodule P concentration. Greater efficiency of ANT22 and E295 than G19833 in utilizing P from rock phosphate and/or organic matter at low P while reliant on N₂ fixation led to a study of root/soil/microbe interaction in the three genotypes. ANT22 reacted to P deficiency by reducing rhizosphere pH. When N₂ dependent, the root acid phosphatase secretion was significantly lower in G19833 than in E295, and ANT22 and E295 had significantly longer root hair than G19833 at low P. No organic acids could be detected in drainage water from any of the genotypes. Nodule occupancy was affected by both genotype and P level. About two thirds of the nodules were occupied by R. etli. However, strains belonging to minority clusters appear to be more low P tolerant than R. etli. At low P, rhizosphere soil from ANT22 contained fewer colony forming units (cfu) but a higher percent of colonies had phosphate solubilizing activity than for G19833, which may have increased P nutrition of this genotype. The study underscores the importance of evaluating genotypes for low P tolerance in different growth systems and various soil conditions. It also shows the importance of including studies of interaction between genotypes, rhizobia, and other microorganisms in the rhizosphere that may affect P and N nutrition and plant growth.